Wide-band phase shifting means



Feb. 2, 1954 F. w. FRINK WIDE-BAND PHASE SHIFTING MEANS Filed Aug. 20, 1946 8 Sheets-Sheet l j .0 a U ,M A .m R J 1]. W .w .r E 3 ,Ml m w. f. a e 0R N m mm m @N27 NA; Awm N/.aa M Mu @fu M@ o Bw .5 M M v, v A m w m a f. E f @o F 2 F. V 9W, M M J R l Q a EC .y 0 4 m/ M INVENTOR fEDER/(A [MFR/NK BY ATTORNE Feb. 2, 1954 F, w, FRlNK 2,668,238 WIDE-BAND PHASE SHIFTING MEANS Filed Aug. 20, 1946 8 Sheets-Sheet 2 F/fuf/vc Y- cyafyffc ATTORNEYS Feb. 2, 1954 F. w. FRINK WIDE-BAND PHASE SHIFTING MEANS 8 Sheets-Sheet 3 Filed Aug. 20, 1946 lNvENToR Fernse/(ar m ,fR/'wr ATTORNEYS Feb, 2, 1954` F. w. FRINK WIDE-BAND PHASE SHIFTING MEANS 8 Sheets-Sheet 4 v', u

' Filed Aug. 20, 1946 Q U T n my mW vx .ml I- m BY Z/wyfi, @hun l ZMA7.

ATTO R N EY Feb. 2, 1954 Filed Aug. 20, 1946 F. W. FRINK WIDE-BAND PHASE SHIFTING MEANS 8 Sheets-Sheet 5 OUTPUT' INVENTOR FREER/C/f 14./ FRI/VK ATTORNEYS v Feb. 2, 1954 F, W ,.-mNK 2,668,238

WIDE-BAND PHASE SHIFTING MEANS Filed Aug. 20, 1946 .8 Sheets-Sheet 6 INVENTOR fet-05mm n4 ,fR/NK ATTORNEYS Feb- 2, 1954 F. w. FRINK WIDE-BAND PHASE SHIFTING MEANS s sheets-sneek '7 Filed Aug. 20, 1946 ATTORNEYS F. W. F RIN K WIDE-BAND PHASE SHIFTING MEANS Feb. z, 1954 8 Sheets-Sheet 8 Filed Aug. 20, 1946 whqrlo Qm,

Patented Feb. 2, 159.54

UNITED STATES PATENT OFFICE 2,668,238 WIDE-BAND PHASE SHIFTING MEANS Frederick W. Frink, East Orange, N. J. Application August 20, 1946, Serial No. 691,852

(Cl. 25o-27) 33 Claims.

My invention relates to phase shlfters in which the amount of phase shift is substantially independent of frequency. More particularly, it relates to such phase shifters in which the original relative voltage magnitudes are maintained for practical purposes throughout a substantial frequency range at the same time producing a phase shift of some constant predetermined angle such as 90 degrees.

An object of my invention is to apply such an improved phase shifter to a system for producing single side band modulation of a carrier frequency oscillator.

A further object of my invention is to provide an improved phase shifter having greater constancy of phase shift and output voltage in the presence of varying frequency than has been heretofore known.

' From another aspect, my invention includes the application of stabilized negative impedances to produce a phase shifter of the type described.

Referring to the drawings,

Fig. 1 is a diagrammatic representation of a single side band modulator employing a 90 phase shifter of a type contemplated by the invention;

Fig. 2 is a circuit diagram of a network including an amplifier which may be used in practicing the invention;

Fig. 3 is a circuit diagram illustrating a method of using negative capacitance to obtain a 90 phase shift;

Figs. 4a, 4b, 5a and 5b illustrating certain of the in Fig. 3;

Fig. 6 is a graph illustrating the voltage frequency characteristics of an amplifier network of Fig. 3 which produces 90 phase shift;

Fig. '7 is a circuit diagram of a compensated network for producing 90 phase shift;

Fig. 8 is a graph showing the voltage-frequency characteristics ofthe network shown in Fig- '7;

Fig. 9 is a circuit diagram of a more elaborate network which produces a greater degree of constancy in output voltage and phase shift than the network of Fig. 7;

Figs. 10a, 10b, 10c and 10d are circuit diagrams illustrating a group of alternate networks each of which will produce a 90 phase shift throughout a range of frequencies;

Fig. 11 is a circuit diagram of another form of phase shifting network;

Figs. 12a and 12b are vector diagrams of certain voltages of Fig. 11 used shift;

Fig. 13 is a circuit diagram showing a simplilied form of phase shifter;

are vector diagrams voltage relationships to produce a phase Fig. 14 is a circuit diagram illustrating an inductively coupled Vbasic network for producing phase shift;

Fig. 15 is a circuit diagram showing a compensated phase shifter using the basic net= work of Fig. 14;

Fig. 16 is a circuit diagram -which shows a more elaborate form of compensated phase shifter likewise using the basic network of Fig. 14.

Referring to Fig. 1, a method of producing single-sideband modulation from a source of carrier frequencies is illustrated.

This method of side-band suppression is described in the November 1945 issue of Electronics in an article by M. A. Honnell at page 166. This method of side-band suppression avoids the necessity for sharply tuned filters. Two separate balanced modulators are used, and the carrier voltages applied to these two modulators are 90 degrees out of phase with each other; also, the audio-frequqency modulating voltages applied to the two modulators are 90 degrees out of phase with each other.

By using the equations for amplitude-modulated signals, Honnell shows that the outputs of the balanced modulators are of the form G sin (21rfct-i-21rfst) +G sin (ZNet-2nighand G is the modulator conversion gain, fc is the carrier frequency, fs is the frequency of the modulating signal.

When these two outputs are added togetherin the mixer amplifier the resultant is the upper sideband, and if they are subtracted the resultant is the lower sideband.

The degree of effectiveness of side band suppression is largely dependent on the ability of the 90 phase Shifters to produce an accurate and constant 90 shift without changing the relative amplitudes of the voltages. These are the phase Shifters I-Ul and I02 shown in Fig. 1. The mixer-amplifier |-3 is a linear amplifier. Energy from the carrier source l-d having a voltage which may be expressed as.E sin A is applied to the input of balanced modulator 1 9'5. Energy from the modulating signal source l-El is first passed through 90 phase shifter l-l. The original expression for the voltage' of the modulating signal source I-96 may be V sin B, where V represents maximum amplitude and B represents the instantaneous phase angle. The

effect of passing through the 90 phase shifter l-UI changes this expression to V cos B assum- 'ing that there is no change in amplitude. rIhe carrier voltage E sin A and the shifted modulating voltage V cos B are applied to balanced modulator l-05 yielding a resultant voltage which, by applying the conventional trigonometric formulas, maybe expressed GEvtsm (A+B) i-sm (fi-Bi] G representing the modulator conversion gain.'

Another portion of energy froniucarrier ysource I-U4 is taken through the 90 phase shifter i-Z where its original expression E sin A vis changed to E cos A. The modulating signal voltage of the form V sin B is taken directly from the signal source l-BG and applied simultaneously 4with the shifted carrier voltage E cos A to the input ofbalanced modulator .1451... ,The output of the modulator may be similarly expressed GEVISin (A+B.) -sin (eA-B) tiyernagnitudes and in shifted phase angle with respect to their inputs throughout the range of frequencies which it is proposed to utilize during transmission, Of course, the phase shifter l-, being connectedin the circuit of carrier source Lelli operates etsubstautielly constant frequency and constant voltage and thus requires no unusualcharacteristics- AIii the ease of Shifter i-t i, however! a milch. more difficult problem is pre- Senediei .which the invention presents a. workable and practical solution,

Inpracticing several embodiments of the invention, negative impedances are used and before proceeding withl a more detailed description of the specific phase Shifters tobe employed, the method of obtaining such negative impedances will be discussed.

In the July 1945 issue of Electronics, page 140, E. L. Qinzton shows how a negative inductance or negative capacitance can be obtained by means of a two stage amplifier having either an inductor or acapacitor respectively, connected from one of the ampliiier input terminals to one of its output terminals. A negative inductance is a reactive circuit element which resembles an ordinary inductance in the sense that its reactance increases directly in proportion to the frequency but it differs from the ordinary inductance in that its phase angle is opposite to that ofV a positive inductance, the vectorial equivalent of multiplication by (-1) and thus is 90 instead of +90. Thus the reactance of such a negative inductance is therefore -iwL instead of -I-iwL as in the case of an ordinary or positive inductance.

Similarly, the reactance of the negative capacitance is -l-/wC instead of 3i/5C as in the case of an ordinary capacitance and moreover the current in the negative capacitance lags the applied voltage by 90 instead of leading it. The

foieeoiuetliei the out1 absolute magnitudes of reactance are the same for either positive or negative inductances or capacitances.

A circuit for obtaining negative capacitative reactance is shown in Fig. 2. The triodes 2-0l and 2-U2 constitute a two stage voltage amplier and triode 2-93 `is connecij,ve d l as a cathode follower which serves to reduce the output impedance. The circuit constants areso chosen that within the frequency range of 200 to 3,000 cycles, for example, the over-all phase shift will be sufficiently small to jbe negligible. The magnitude of phase shift produced by the amplier is further reduced by the use of an inverse feed-back circuit whichornay be traced thorugh resistor 2A Vand resistorA2-S5 to ground, resistor 2-05 thus forming a common circuit element in the cathode return circuits of triodes 2-01 and 2-03. This inverse feed-back connection also serves to stabilize the overall gain of .the amplifier and ref:- move or reduce the effects ofvariations in tube characteristics. Assuming that the amount-of phase shift has been reduced to a negligible value, the A. C. voltage E2 between the cathode of tube 2-53 and ground will be in lphase (relaf tive to ground) with the A, C. voltage E1 applied to the grid of input tube 2-01, `The currentI which flows in the capacitor 2-06 as a result of the diference between these two voltages leads the voltage E2 by 90 and hence lags the input voltage E1 by the same amount, since E1 is op! posed in direction to Eg. Because this vlagging current increases in magnitude, in direct propor-A tion to the frequency .of the voltage E, the overall effect presented to the generator 2-01 at terminals 2-D8 and 2-09 is that of a negative capace,- itance. In other Words, the reactance presented by terminals Z-Dg and 2439 is inversely proportional to the frequency of the applied voltage, However, unlike the usual or positive capacitance its phase angle is reversed and a lagging current flows instead of a leading current.

In ordinary usage, the term negative'react ance iS generally understood toV referv to a conventional reactance ofV a capacitative character whose magnitude decreases with increasing frequency and whose phase angle is E 2 as indicated by the `familiar expression In a negative capacitative reactance, the vreact--` ance is capacitative in character, inasmuch as its magnitude decreases with increasing frequency, but its phase angle is reversed to 1T it as indicated by the expression X=,+:i/.wC. A similar situation prevails with respect to ,inductive reactance, the conventional reactance being -l-iwL and therefore being referred to as positive.

In order to avoid possible confusion between conventional reectemces andthe special negative reactances, deribed above, and particularly where it is immaterial whether the reactance is inductive or V3dpactat,ive in character, these special reactances which' are obtained by the use of ampliiication as shown in Figure 2. will be referred to as reversed reactances, since their characteristics areV otherwise the sameV as those of their conventional and' Well known counterparts, except that their phase anglesare reversed. Similarly, the reactanceof the simple capacitor vor inductor willbereferred to as conventional reactance, in order to emphasize the distinction between one type of reactance and the other. The expression reversed impedance will be used to designate an impedance comprising a reversed reactance.

In accordance with Ginztons analysis the quantitative value of the negative capacitance may be evaluated from the equation -Cn is equal to CM1-A1) where C1 is the capacitance of capacitor 2-06, -Cn is the effective negative capacitance presented to generator 2-01 at terminals 2-08 and 2-09, and A1 is the net overall gain of the amplifier including the effect of inverse feedback through the common cathode resistor 2-05. This expression does not take into consideration the effect which the' resistor 2-04 might have on the magitude and prase of the current flowing in capacitor 2-06, butthe circuit constants are so selected that the effect 'of' re-` sistance 2 04 is negligible as compared with the reactance of capacitor 2-06.

It is apparent that if an inductor were. substituted for capacitor 2-06 the effective input impedance of the network would be a negative inductive reactance instead citive reactance. f Referring to Fig. 3, a circuit is illustrated in which a reversed impedance network as shown in Fig. 2 is utilized to obtain a 90 phaseshift within the frequency range from 200 to 3,000 cycles, for example. Fig. 3 is the same as Fig. 2 except that resistors 3-0I and 3-02 and capacitor 3-03 have been added. As previously explained in connection with Fig. 2, capacitor 2-06 appears at the input circuit as a negative capacitor connected from the grid of triode 2 0! to ground and is the equivalent, from a circuit performance standpoint, of a negative capacitance represented by the dotted lines 3-04. For the purposes of computation we may, therefore, assume that we have a negative capacitance shown dotted 3-04 connected between the terminal 3-05 and ground.

At the input circuit of the phase shifter, the resistor 30I is so selected that at a frequency of 750 cycles its resistance is equal in magnitude to the reactance of the equivalent negative capacitance 3-04. A frequency of '750 cycles has been selected to facilitate computation and because it is the approximate geometric mean between the frequencies of 200 cycles per sec. and 3,000 cycles per secr., these frequencies being the upper and lower limits of the frequencies arbitrarily selected for the particular phase Shifters used as examples. and resistor 3-02, which resistor is very much larger than the cathode resistor 2-04. have been chosen so that at the mean frequency of 750 cycles the net output voltage E4 is 45 the total output voltage Ea appearing between the cathode of triode 2-03 and ground.

The other circuit constants have been so selected that the only phase shifts which may not be neglected are those produced by resistor 3-0I and equivalent negative capacitor 3- 04 at the input of the phase shifter and the output capacitor 3-03 and output resistor 3-02. Referring to Figs. 4a and 4b, the voltage of source 3-06, E1 is equal to the vector sum of the voltage E2 across the input terminal 3-05 to ground and the voltage drop Ea-oi across resistor 3 0! at themean frequency of 750 cycles per sec.l The voltage across resistor 3-Ill` lags 'of a negative capa- The values of capacitor 3-ll3V out of phase with- Uli the input voltage E1 because of the fact that the current for the equivalent negatieve ycapacitance 3-04 ista laggingcurrent. The voltage E2 leads voltage Eisince it is the voltage across the negative capacitance'and must, therefore, lead the current.

Fig. 14h shows the usual case of a resistor in series with an ordinary capacitor. Now assume that thefrequency changes to some new' value', f, Which'differsfrom the mean frequency of 750 cycles. The new vector relations are shown in Figs. 5a and 5b. The voltage ratio at the input Ea-m/Ez is now equal to f/750 instead of to unity, and at the output'the new ratio Erma/E4 is'equal to 750/f instead of unity. At the input, the phaseangle therefore, @N92- 490 frequency f. Hence the signal voltage in passing through the network of Fig. 3 from generator 3-06 to output resistor v3*-02 is first shifted in phase by an angle 491 produced by the combined action of resistor 3 0! and the equivalent input capacitance regardless of the value of i-0d and then at the output it is shifted through a complementary angle 02 by the combined action of capacitor 3-f3 and resistor 3-02 and the total phase shift will be regardless of the input frequency. The network of Fig. 3, as has been shown, will provide the required constant 90 phase shift but it isnecessary to provide additional circuit elements before the proper frequency-amplitude relatons will be maintained. Y

Considering the network of Fig. 3, let A1=ratio of Es/Ez, which may be considered constant, because resistors 2-04 and 2-05 are very small compared with resistor 3-02. Then,

1 A 1 441/1 +mn2 @JQ/1 +tan 02) but according to Equations 1 and 2,

Then,

Values of 'Et/Eicorrespond'fmgto variousv .values of frequency j' have. been calculated. by Tllirls 0f Equation 3, and' have been plotted: in Fie". 6. As' will be noted'- from -`an examination of Fig. `6,'a suitablee equalizingnetwork:` the necessary amplitude correction provided, however, that this can be done without alteng'pre-'existe' ing phase relationships.

zSeveral different networksV were-devised for equalizing the gain over the required frequency range without impairing the accuracy' ofthe 90" phase shift obtained. Thezsimplest of these networks, and one whichgives good'enough results for some applications, is shown in Fig. 7, which includes both the phase shifter and the ampli-= tude compensating network.;` Triodes 'I-lll, 1-02 and 1-93, amr the'rrassociatetl resistors and capacitors, comprise a phase shifter which is the same` as Y that shown in 3 except 'thattriod'e's 1-92 and 1-03 are the two trode sections ora twin triode, irisiaeaclrfL being' completely" separate tubesas in' Fig. 3. The voltage-supplied tcBalanced Modulator vI-olc. l is obtainedaerossy resistor' -l-'I Soif Fier-A ForA amplitude compensation; `a 's,econd-90 phaseshiter; ern-playingx triodeel-Mi" 'M95 and i408 has been added, 'and is connected cascade' with therst-phase shiftergalsoythe outputvolt ageY of'- this second *phase* -shifter isu connected through resistor 1-09 to resistor 1-I0,\whic1fx"is inf-the input circuit of :the'ilrst rphasshiffter. Sincevvthe overall phase-1 shift from the secondary wimlm-oi transformer "FW tovfresisto'r'flel is 1'80 degrees; .the :feedback produced -"by resistors 1-09 and 1-10 is an invers'eefeedback. t-op'-l pesesrthe .uoltage ofitransformerf'l-l El, xbutwit bdoes notchangerithei phasesoff-any' `ofthe voltagesfin` the'network.n

Let Arzgain from the grido 'T- ritothemcathw ode of 1-33, Az=gain from the grid of l-04 to the cathode of, LR6- Them'withreierenceto Equaef.

tion 3 above, it is evident thatwhen resistor i-DB is disconnected, the overall gain of the entire network yfromtransformer 1-II to resistor T-fis' voltage gain represented by A3 from resistor 1-. IE to resistor 1 08 is:

g. lshingzthe-feedback circuit through resistorm wesmust introduce a nearv factor-,f

' 'rR'zL1o/(R'z`-1o-1-Ri-.os whichimay flue-called the feedback: factor; The overall gain.- of; the; :network `from transformer 'l-l I toresistor 1-08 isrnow where AF' is'the overall gain including the effect of the feedback circuit.

The next step in our analysis is to .determine howthe amplitude of the output voltage,obtained across resistor 'l-I Svaries Wthvthe frequency. By. .referring to' Equation 3, 'we can see .thatthe where To ndthe gain from. transformer 'l--Hy -to `:resistor1-i 6. we Vmust divide Equation 5 by llilquation 6, thusobtaining sisters` :1 -.B91 and 'I-ly'so vthat effi-17.8. Then, Equation 7 lbecomes is f The :values of correspondingY to variousfrequencies have been and value :throughout the .,200 to 3,000.: cycle frequency column of Table I, and then dividing by the difference between 95.84 and the same ligure. For example, if the modulating frequency is 250 cycles/sec. the ratio of the desired sideband voltage to the rejected sideband voltage is 98.38+95.84 98.38-95.84-76'46 The ratios for the other` modulating frequencies have been calculated, and are recorded in the fifth column of Table I. The corresponding decibel values have been recorded in the sixth column.

Table I Relative sideband Afm/43K Amplitude K Frequency, n

Cycles/Sec.' Iellfzent N l o axiumerica Value mum Ratio Db Value v Figure 9 shows a more elaborate network which provides for a much greater rejection of the undesired sideband than the network shown in liig. 7. In Fig. 9,'triodes 9-'|ll, 9-02, and 9-03, with their associated resistors and capacitors, constitute a 90degree phase-shifting network similar to that s hown in Fig. 3. This phaseshifting network is followed by a triode amplilfier 9-04, and the output voltage for application to the balanced modulator l05 of Fig. 1 is obtained from the plate circuit of tube 9-04.

Amplifier 9 0!! is designed to have inverse feedback from its plate circuit to its cathode circuit, but the feedback voltage has to travel through triodes 9-05, 9-06, 9-01, 9-08, 9-09, 9-I0, 9-II, and 9-l2, in passing from the plate circuit of triode 9-04 to the cathode circuit resistor 9-l5.. In' passing through these various triodes and their associated circuits, the feedback voltage undergoes an. amplification, theV magnitude of which is a function of frequency. -The gain of amplifier 9-04 .is governed by this feedback voltage in such a manner as to compensate for the frequency discrimination occurring in the phase shifter composed Voftriocgles Q -0l, 9-02, and $93.] Since the variation offamplitu'de in the output of this phase shifter is as shown by the curveof Fig. 6.amplier 9-04 must have a characteristic in which the ordinatesare proportional toQthe rcciprocals of those plotted in Fig. 6.

In the feedback network associated with amplifier 9-05 there are two complete QO-degree phase Shifters in cascade. One o f these phase shifters employs triodes 9-06, 9-01, endg-08j and the other employs triodes 945B, S-lll, and 9 1 l. The feedback network also includes input and output cathode followers, 9-05 and 9-I2` respectviely, which are used for impedance transformation. Since each ofthese phase shiftersis similar to that shown in Fig. 3; the gain of veach is rep- I10 resented by an equation similar to Equation 3; i. e.,

E `1Q K11" En* 1 -I- n2 and 13 1 +712 Where and Kn and Kmare constants.

Then, the overall gain, with feedback resistor 9-l6 disconnected, is

. v Y l n 2 K 1+n2) E' n 2 v v. u 1+2K 1+n2 Y where )92 is the `feedback factor, whose magnitude is dependent on Rsi-1s. In this application,

,B2 is adjusted (by adjusting resistor 9-l6) so that /32K1s:2.7. Then n 2 K 1+ni E11 n 2 1+2.7 m) Voltage E14 is not all applied to the cathode circuit of triode 9-04, but is first reduced in the ratio Ril-15 Rs-iH-Rs-u v If the feedback factor of amplifier triode 9-04 Rsi-15 KM- (R9-15+ Riv-14 K13 If the gain of amplifier 9-04 without inverse feedback is Ai, then the gain with inverse feed- 11 If fue adjust." the wallie.; :.jof'V .Knee-.z by: 'adjusting R19-15 S0 that K14At:23.97, then A. a (nte) Equation shows how the gain of amplifier it is connected n AF Mure) Where Kis is a constant.

Let Aazoverall. gain vfrom..transformer 9-I9 to the outputrircuitso amplifier 9404. Then,

Where K1@ is a constant.

. In order to show how the overall gain varies vvvfith frequency, yvaluesof AaK 1s` have beencalculated Yfor various frequencies; `land have been recorded inthe'third'columrr of Table II. The fourth column gives the same results expressed as percentages of themaximnm value.

Let us suppose"that the .modulating voltage applied to balanced modulator V`1 05 of Fig. l varies With frequency as indicated in the fourth column .of Table .IL while. the. voltage applied. to balanced modulator l-Ulis independent -of irequency, and is equal to 99.56% of the maximum applied to balanced modulator 1 05. The ratio of the amplitude of'thedesired sideband to the amplitude of the undesired sideband is then given v-by the expression:

Where Aa/Kis is 'given in per cent ot maximum. 'Ihis-ratioV has beencalculatedoeyarious frequencies, and has been recorded in the iith column :of Table II. The.corresponding decibel values are given in the sixth column. It can be seen that the minimum attenuation of the undesired sidebandA cccurslat about 375 and 1500 cycles/sec., and .is 52.89 decibels. This seems sufficient for preventingdnterference .on the adjacent channel, even withoutlthehelpo anoutput filter.

The circuits of Fig. 9 are more complicated than those of Fig. 7, but they' give a greater degree of rejection of the undesired sideband, as can be seen'fbyicomparing the last column of Table II with. the last column of Table I, which applies toithe circuits of Fig. 7.

Table I shows thatithewlowest ratio of sidebandsuppression .which is obtained in the 187.5- 3000 cycles/sec. range is about 33 db. If the singlet-.sideband generator is: to be used in a ca-rrier-currenttelephonesystem for the purpose of obtaining aimaximum number ofir commun-ication channels,1thisamountfofy rejection is hardly enough, as it will stilll allow a noticeable amount of interference on the adjacent channel.

*Oneimethod ofimproving theioperation would .be to :obtain the sideband :rejection partly 'by'bab '.lancingout the undesired' sideband and .partly by .ajlterfthat introducesaabout 18 db additional attenuation infthe undesired"v sideband, thus .,bringingfthe total :attenuation up to 5l-db; -Obviouslm such afdter wouldnot have tobeas .complicatedror expensive as 'the' lters-ordinariiy used in singleesidebandcommunication. Intsome cases, -.additiumal- .sideband' rejection might. be.=obtained.by.using a pairedcoupled resonant; circuitsin the input of. theemixerfamplifier of Figi, and another pair ofnsuch circuits in. the .output .of thismmixereampliier.

When single-sideband transmission isusedior short-wave.fradiotelephony. the. situation is .considerablydifferent,v becausefthe purpose ofusing single-sideband transmission insuchapplications is not necessarily'. that -ofincreasing the number of communicationehannels ayailable....1n .some cases,. the .principal `purposes are tov improve.` the signal-to-noise ratio .obtainable with a given transmitter. power.. and. to prevent certain.l types of distortionithat result from .multi-path transmissiony For. these purposes the .amount .ot sideband rejectiomindicatedin"Table'. I might be entirely adequate, evenif. nojlter of .any kind were. used for. attenuating .the .undesired sideband.

The complete 'elimination of filters is quite advantageous, particularly in. short-wave .applications, because the design of :filters having a sharp-enough cutoff .becomes increasingly .dimcult as the, carrier.r frequency is increased; so that in short-waveapplications employing lters itfis. usually necessary to. use .-.three modulator stages.. each' of. .whichthas .to .have av sourcey .lof carrier voltagehaving. .airequency different. from that suppliedv tothe other modulatorstages; :furthermoraeach .ofthe three .modulator .stages .has tohave. anoutput. lter, or other selectivenetwork, to j. eliminate the. undesiredsideband. Another. .disadvantage 'of singleesideband .systems employing. filters'. is..` that.. eachiilter has. :to be designedfor aspecic. range .of frequencies.. ,and cannot be quickly and easily adapted tov a different frequency. range. Thesystem shown in Eig. 1 can apparently. be applied. as readily.- to .short- Wave transmitters :as toflow-frequency .transmitters, and does not require any more modulator stages, or other circuits, for short-wave applications than for low-frequency applications. The compensated phase-shifter circuits shown in Fig. 7, together with any other necessary audio-frequency circuits, can be assembled in a standardized unit which would bethe same for all applications, regardless of the carrier frequency.

It is obvious that in the system shown in Fig. 1 a small amount of carrier energy couldbe introduced into the output of the mixer-amplifier and transmitted along with the sideband; and the amplitude of the carrier wave could be reinforced at the receiving station, as is sometimes done, so that there will not be any distortion caused by frequency drift such as might occur if an independent local oscillator were used.

The network shown in Fig. 3 is not the only network that Will produce a 90 phase shift at all frequencies. There are at least eight such networks, of which four are represented in the simplified diagrams of Fig. 10. In Fig. 10A we y have the same network as shown in Fig. 3. In Fig. 10B, positive and lnegative inductance are used, instead of positive and negative capacitance. In Fig. 10C a negative capacitance and a positive inductance are used, and in Fig. 10D we also have a negative capacitance and a positive inductance, but in a different arrangement from that shown in Fig. 10C.

From the four networks shown in Figs. 10A to 10D it is possible to derive four additional networks for the same purpose, by interchanging the algebraic signs of the reactive elements in each network. For example, in Fig. 10A we would change Cn to a positive capacitance, while C would be changed to a negative` capacitance. In any of the eight networks mentioned, it is necessary to choose the circuit constants so that the frequency at which a 45 phase shift occurs in the first resistance-reactance combination is the same as the frequency at which a 45 phase shift occurs in the second resistance-reactance combination.

In the phase-shifter of Fig. 3, triodes 2-0I, 2-02 and 2-03 are used not only for producing an equivalent negative capacitance 3-04, but also for amplifying the signal voltage before applying it to capacitor 303 and resistor 3`02. Instead of this, it is possible for the signal voltage across 3-04 to be applied to a separate amplifier, and the series output combination of capacitor 3Il3 and resistor 3-02 can then be connected to the output of the separate amplifier instead of to the output of triodeV 2-03. In such an arrangement, the sole purpose of triodes 2-0I 2-02, and 2-03 would be to generate a negative capacitance. When triodes 2-0I, 2-02 and 2-03 are used solely for generating a negative capacitance, it is possible to arrange this set of tubes, together with the signal source and the signal amplier, in such a manner that the negative capacitance (or negative inductance) is introduced into the network as a series element, as shown in Fig. 10D, instead of as a shunt element as in Fig. 3.

In each of the four networks shown in Figs. 10A, 10B, 10C, and 10D, the first resistancereactance combination is separated from the, second by an amplier. This amplifier is not essential, provided that the second' resistancereactance combination can be coupled loosely enough to the first combination so that it does .not appreciably load the first combination.

In all of the 90-degree phase-shift networks voltages applied to lil discussed so far, the phase shift is aecompnshed in two steps: 'I'he signal is first shifted inphase by one resistance-reactance combination, and is later shifted in phase by another resistancereactance combination, and the -total phase-shift is degrees.

Fig. 11 shows a 90-degree phase shifter which differs from the preceding phase Shifters in that the two resistance-reactance combinations are connected in parallel. The voltage across capacitor IIIJ5 is applied between the grid and cathode of triode II-03, and the voltage across equivalent negative capacitance II-U'I is passed through phase inverter IILIJI and then applied between the grid and cathode of triode I I-02. The resultant output voltage of triode I I-02 and triode I I-Il3, whose plates are connected in multiple, is proportional to the differencev between the voltage across capacitork II-05 and the voltage across equivalent negative capacitance I I 01.

Fig. 12A is a vector diagram which shows the phase relations existing in the circuit of Fig. 11 at a frequency of 750 cycles/sec. assuming that the circuit constants have been so chosen that at this frequency the absolute magnitudes of the voltage drops across the four circuit elements II-M, II-05, II-IJG and Il -Ul are all equal. The output voltage is proportional to vector Eo. When the frequency is increased to some value f, then and as shown in Fig. 12B. Now,

E11-04=Sill 6 Y and Then,

Instead of using positive and negative capaci- I tances in Fig. 1l, it would have been possible to use positive and negative inductances; furthervmore,'the positions of the resistances and capacitances could have been interchanged, so that the yresistances would be on the ground side of the circuit.

Fig. 13 shows a phasel shifter in which the thetwo resistance-reactance combina-tions arel degrees outer phase.- fl-his arrangement z makes it unnecessary vto use l-tl1e phase inverter triode Iii-|. shown in Figi 1l-.

.In Figs.. 11and13 thee method ofl obtaining the. negative capacitance was not shown, but circuits such as that of Fig. 2 could `beusedfor this purpose.

Either .of these phase Shifters may be usedinstead of thephase-shifter of Fig. 3,-with the same type of amplitude compensation.

A question that-naturally arises inv connection with-the circuits shown in Figs. 'l and. 9 is whether they will-get-out of adjustment too easily, due to changes-in .amplification vcaused by variations in tube characteristics. In this connection. it should be pointed outthat .all of the amplifiers in Figs. land 9` have inverse `feedback insome form, .and it iswell known that if enough inverse feedback is used, the gain of an amplier can be made largelyindependent of .the tube characteristics, and it will. depend almost entirely cn the vconstants of. the feedbacknetwork. rvIf the feedback networkA is made `up* of resistors and capacitors, vthe stability lof thefampliergain is comparable tothe `stability of these resistors and capacitors.

Fig. J1&1 shows abasic. network for producing a 90 phaseshift. Thisnetwork is s hownfbyjlvl.` A. Honnell `in the article. published. in -l\lox e1nber 1945 Electronics.Y

If resistor M-l is made very large compared with the reactancesof" capacitor l4-02 and transformer lil-04 (e. g., 100 timesas large or larger), the output voltage Ez will be very nearly-4 90 degrees out of phase with the input voltage E1. The current through resistor ill-iii, capacitor lil-B2, and the primary winding lli-03 of transformer i4-04 will remain substantially constant over a wide range of frequencies (because the total impedancepfthecircuitis governed primarily by the value of resistor M-i which is large), so that the magnitude of voltage Es may be considered to be directly proportional to the frequency, while the magnitude of voltage Ec is inversely proportional tothe frequency. The secondary winding of transformer Ill-D4 is` connected with such polarity that the Voltage ES is added in phase to the'fvoltage Ec.

Let us suppose that the circuit constants have been 'chosen so that at a frequency of 750 cycles/ sec.' Ezhas some. value..En,. and Es=Ec=Eo/2. Then, if the frequency is changed to some other value j, the new value of E2 is given by the `ex' pression, E Eb E 14- 2 0 1 0 'It EE-*Z-.n-i- 2 (1J-( 'n where n=f/750.

It is obvious that-.the magnitude of E2 will vary greatly if the frequency Lis varied over the range from 20D-3000 cycles/sec.

By comparing Equation 12 with my Equation 3, one can compa-re the amplitude variation occurr-ing in the lnetwork of. my.' Fig. 14 with the amplitude variation occurring in the network of my Fig. 3. It. cany bessen that the amplitude obtained in one case is proportional to the reciprocal of thatobtainedin the other case.

The variation in amplitude can be greatly reduced by elaboratingon thecircuit. shownin my Fig, V14, and one pensation isfshown in my Fig-15'.'

This Acomprises two basic. phaseshifting networks |50i and I5--02 and three associated two.- stage amplifiers .i5-03, .l5-04 .and 15-05; `Prinsmelly. the, phase` shiftgproducedl by network method of .producing such ,com-

`IBAH,sis.-utilized, the other network .l5-D2 Aforming a partofV the amplitude-frequency compensa* tion circuit. VInverse-feefsl-back is provided via conductor `I 5-0 6.

VIf the feedback connection 15-06 shown in Fig. 15 is not used-We may, in accordance with Equation 12, write,

l-i-n2 YEnum-K1( n (13) whereKi is a constant; A

E12-K2( n 14) also,v Where Kz isA another constant; then,

@einem 1+n2 2 Eri-En Exc-KiKz( n (15) If the feedback connection I5-06 is used,

2 2 E KiKzCJfn) To find'Em/En under this condition, we must divideEquation 16 by Equation 14; i. e.,

facing) A. HKXKZCJ Assume that the amount 'of amplication is adjusted sothat Y1f1"1Is2-1/7.8. Then,

'r2-Haart If we multiply both numerator and denominator of this'expression by (fl-ttf Equation 19 hasithe same form as Equation 8, which applies to the network shown in my Fig. '7. Evidently, the output amplitude .variation with respect to frequency is the same for the circuits of Fig. 15- as for the circuitsv of Fig. 7. rhis Ivariation ,has been shown in Table I and Since thereis a rather large loss of audiofrequency voltage in the phase Shifters of Fig. 15, due to the necessary use of large series resistors, a large amount of amplification is required to make up for this loss. Moreover, each of the three ampliersshown in Fig. 15 should have a considerable amount of inverse feedback, to stabilize the gain. An alternative method of Acompensating for amplitudevariation is shown in Fig. 16. 1n Fig. lthe vphase shift is principally produced by basic network I6`0i and its associated input amplier Iii-04; the remainder of the system pro- 17 viding compensation for amplitude variation. Amplifiers |6-0'l and lli-08 are identical with amplier lli-06 which is shown in detail. The ratio of E22 to E21 in passing through amplifier lli-| can be found by referring to Equation 12:

en 1 +12) m42( 7L where Kzi is a constant.

Amplifier lli-05 is used to compensate for the amplitude variation produced by phase shifter IG-Ul. Amplifier lli-05 has a feedback system consisting of amplifiers I 6-06, lli-01, and |6-98, and phase shifters lli-02 and lli-03. This feedback system receives Voltage from the plate circuit of amplifier l6-05, through resistor |6-(i9, and delivers voltage to the cathode circuit of amplifier |6-05. The overall phase shift in this feedback system is 180 degrees, due to phase shifters lli-92 and lli-03 There is a feedback connection lli-I0, from the output of lli-98 to the input of lli-06.

From Equation 12, we know that where Kzz and Km are constants. If feedback connection lli-I0 is omitted,

Where K24' a constant which. depends on the amount of feedback coupling between amplifiers lli-08 and I 6-06. Let us assume that the circuits are so adjusted that the product K24K22K23=9 Then,

, 1 2 2 ELKZZKN( when the feedback system comprising lli-06, lli-02, lli-01,1643, and lli-08, is used, is given as where Kzs is a constant that depends on the magnitude of resistor lli-09. Substituting the value of Ezs/Eziobtained from Equation 24 in Equation Assume that Ka, is adjusted, by adjusting re-Y mu1tip1y Eze/E21, as obtained fro To obtain the overall gain, Egg/E21, we must m Equation 20, by Eza/Ezz, as obtained from Equation 26; i. e.,

E21 E21 E22* 1-1-8 v greatly reduces'the variationin output amplitude Y with frequency.

sister ls-os so that the product K26K22K23K25=a 7% Table III Amplitude Frequency (cycles/sec.) n (percent of maximum) believe to be thebest I Vhave described what I embodimentsof my invention. I do not wish,

however, Lto be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. In a network for producing a predetermined substantially constant angle of phase shift throughout a range of frequencies and having input and output connections, in combination, angle producing means connected intermediate said input and output connections for producing the predetermined angle of phase shift but which incidentally introduces distortion, amplifying means connected intermediate said input and output connections and in cascade relationship with respect to said angle producing means and including a negative feed back path, and compensating means included in said feed back path and disposed to compensate throughout said range of frequencies for the distortion produced by said angle producing means without appreciably aifecting the magnitude of the angle produced.v

2. A network as in claim 1 in .which the angle producing means includes a reversed reactance.

3. A network as in claim 1. in which the angle producing means inclu nected-to produce at least a part of the angle.

4. Anetwork as in claim 3 in which the transformer comprises first and second windings each winding lhaving two terminals, the rst winding being serially included in a series circuit comprising a resistor and a capacitor, one of its terminals being connected to the capacitonthe resistor being of relatively large'resistance for V- maintaining the impedance of the series circuit des a transformer con' quencies, and the second. winding having one of its terminals connected to said terminal of.- the first winding and to the capacitor, the other terminal of thesecondwinding providing a connection from which the produced angle may be derived.

5. A network as in claim 1 in which the angle producing meansv consistsof phase shifting means.

6. A network as. inA claim 1k in which the compensating means comprises further angle producing means.

'7. A network asin claim 6 in which the further angle producing means is substantially identical with the rst-named angle producing means.

8. A network according to claim 1 in which the compensatingmeans comprises a further network, saidi further network, except for the feed back path thereof, being substantially the samev in alli respects as the first-named network.

9. In a networkfor producing a substantially constant90f phase shiftin a voltage of varying. frequency, an amplifier, a capacitor coupling. an output terminal of the amplifier to. an. input terminal of the same relative instantaneous polarity, a resistor connected in series with the input of said amplifier and having a resistance substantially equal to the reactance of said.. amplifier input ata pre-selected mean frequency, and an output network comprising a resistance and capacitance in series, saidresistance andv capaci-V tance values being such that the angle of phase; shift produced at-theY output of the amplifier willv be-the complement ofthe angle of the'phase shift produced at theV input of the-amplier regardless of the frequency being amplified'.

110. In aA network for producing a substantially constant 9.0"A phase shift in a voltage of varying frequency, an amplifier, a capacitor coupling air output terminal' of the amplifier to an input terminal of the same relative' instantaneous polarity, a resistor connected in series with the input" of said amplifier and having a resistance substantially equal' tothe reactance ci said amplier input at a preselected mean frequency, and

an, output network' comprising a resistancel and capacitance in series; said` resistance and' capacitance values beingsuch that the angle ofphase shift producedpat tliecutputofthelamplilier will be the complement ofthe angle ofthe phase shift produced :at the input' of the amplifier regardless of the frequency beingampliiied, andl e. volt age compensating network designed to restore to their original relative intensities the various frequency components ofv the input voltage without appreciably affecting the two complementary angles of'phase shift introduced by the capacitor, resistor and' output` network.

l1'. In a ninety-degree phase shifting network for producing a.` sulost'anti'allyv constant phase shift throughout, a range, of frequenciesA and. having` input and' output terminals; an amplifier; a. reactive circuit element. couplingA an. output terminalof the amplifierl to an .input terminal. there-- ofi of' the same relative instantaneous. polarity; a resistor connected in, series with, the input of saidampliiier and. having a resistance substan:` tially equal to the reactance of. saidamplier input', at; a.. preselected mean frequency; and a; re-` active. network, comprisingV4 a; resistance: and: al. reactive circuit element, said-resistance andreactr lance values beingl such thattnceangle of phase. shift produced therebywill, be theV complement'.r oftheanglef; phase shift producedlby theicom' tive circuit element andthe first-named resistor; andIv circuit: meansY interconnecting the reactive network. and theA last-named. combinationin. cas`- caded relationshipt intermediate thef input. and output terminalsaof theninety-degree phaseshifting network'.

l2. A ninety-degree phase shifting network asin claim ll further comprising a voltage compensating network interposed between the input and output terminals thereof, the characteristics of said` compensating network being such. as to restore substantially. toA their Yoriginal' relative intensitiesat the outputterminals, the various frequency components. of a voltage of varying frelquency: applied to the` input terminals', without materially affecting themagnitude off the ninety degree phaseshift.

13. Ina system for producing a substantially' constant ninety-degree phase sliif-t at all fred quencies ina particular range, a' network comprising an. amplifier, a reactance element connecting an. output terminallof said amplifier tcian input terminal of the 4same relative instantaneous polarity, and a. resistor connected inl seriesy with the input of said, amplifier and: having a resist- "l ance substantially equal to the reactance of said amplifier at a pre-selected frequency in said range, and another network connected in cascade with the first mentioned network and consisting of a resistance and a react'ance in series, the

n.values of said resistance and. reactance being such that the angle of phase` shift; produced by; said other network is the complement of tlfie angle of phase shift produced by, thesfirst menticned network.

14. In a system for producing. a., substantially constant ninety-degree phase shift at all frequencies in particular range, a network comprising ran amplifier, a reactance element connecting an output terminal of. said. amplifier to an .input terminal of the same relative instantaneous polarity, and a resistor connected in. series'with' the input of. said' amplifier and hav.- ing'a resistance substantially equal" to the re.- actaiicel of said amplifier ata pre-selected frequency in said range, another network connected" in cascade with the-first; mentioned network and consisting of a resistance and a reactance insee ries, the values of said resistance and reactance being such that the anglevof phase shift produced by said other network is the complement of the angle of phase shift-produced bythe first mentioned network, and a voltage compensating network designed to restore to their original relative amplitudes the various. frequency: components ofthe input voltage ofA said. system without apprecialolyv aiiecting the resultant angle of phase'- shift. produced; by.- thefirstv mentioned` networkY and said other network.

l5; In a systemfor` producing-a substantially constant ninety-degree;y phase Shift at all frequencies in a particular range, a network comprising an amplifier, a reactance element con-,- necting arr output terminal" of said' amplifier, to. aninputl terminali of the same relative instan.-

A taneous` polarity; and a resistor connected in.

series with the input of said amplifier and having a resistance substantially equal to thereactance of said amplifier at a pre-selected frequency in said range, another network consisting of a re- ;sistance and,areactance-in-series;,and means for combining the transmission characteristics of both of said networks to obtain a resultant ninety-degree phase shift at all frequencies in binatiomof.l theamplieri; therrst-namedreacsaid range;

` 16. Ina system for producing a substantially constant ninety-degree phase shift at all frequencies in a particular range, a network comprisingfan amplier, a reactance element connecting an output terminal of i said yamplifier to an input terminal of the same relative instantaneous polarity, and a resistor connected in series withthe input of said amplifier and having a resistance substantially equal to the reactance of said amplifier at a pre-selected frequency in said range, another network consisting of a-resistance and a reactance in series, means Vfor combining the transmission characteristics of both of said networks to obtain a resultant ninetydegree phase shift at all frequencies in said range, and a voltage compensating network designed to restore to their original relative amplitudes the various frequency components of the input voltage of said system without appreciably affecting the resultant angle of phase shift produced by the first mentioned network and said other network. 17. In a phase shifting device, in combination:

a first network comprising a resistance and a reversed reactance in series; a second network comprising a resistance and a conventional reactance in series, the angle of phase shift produced by the second network being substantially the complement of the angle of phase shift produced by the first network throughout a wide range of frequencies, and circuit means interconnecting the two networks for combining the transmission characteristics thereof to obtain a resultant ninety-degree phase shift throughout said frequency range. 18. A phase shifting deviceas in claim 17, further comprising a voltage compensating network interconnected with the firstV and second net-V works and with the circuit means, the transmission characteristics of said compensating network being such as to restore toward their original relative intensities the various frequency components of a voltage of varying frequency whose phase has been shifted through an angle of ninety degrees by said device without appreciably affecting said ninety degree angle.

19. In a phase shifting device, in combination: a rst phase shifting network providing a substantially constant ninety-degree angle of phase shift throughout a wide range Vof frequencies; a feedback path connecting the output of said network to the input thereof; and a second phase shifting network having transmission characteristics similar to those of the rst network included in' said feedback path for restoring toward their original relative intensities the various frequency components of a voltage of varying frequency whose phase has been shifted by said device without appreciably affecting said ninety degree angle.

20. In a phase shifting device, in combination: a rst phase shifting network providing a substantially constant angle of phase shift throughout a range of frequencies; a feedback path connecting the output of said network to the input thereof; and a second phase shifting network included in said feedback path and having a substantially constant angle of phase shift which is the supplement of the first-named angle throughout said range of frequencies.

21. In a phase shifting device; a feedback loop comprising an amplifier and two ninety-degree phase shifters, one of said phase Shifters providing equalization for the transmission characteristics of the other; a first circuit means for applying a signal to said feedback loop; and a second circuit means for deriving a signal there- 22 from, the transmission path between said first and second circuit means including the one of said phase Shifters whose transmission characteristics are equalized and excluding the phase shifter which provides said equalization.

22. In a network for producing a substantially constant phase shift in a voltage of varying frequency; a series combination of a resistor and a conventional reactance connected to be energized by the voltage of varying frequency; a seriescombination of a resistor and a reversed reactance of character like the first-named reactance connected to be energized by the voltage of varying frequency in multiple with the rst named combination; means for inverting the phase relationship at the junction between one of the reactances and one of the resistors; and

means for combining the voltage appearing atV the junction between the other reactance and the other resistor with the output of the phase invertingv means whereby a voltage will be derived Whose phase will be shifted by a substantially constant angle of 90 throughout the range of variation of said varying frequency.

23. A network as in claim 22 wherein the character of the two reactances is inductive.

24. A network as in claim 22 wherein the character of the two reactances is capacitative.

25. In a network for producing a substantially constant 90 phase shift in a voltage of varying frequency; the series combination of a resistor, a conventional reactance, a reversed reactance of character like the first-named reactance, and a second resistor, in the order named, arranged to be energized by the voltage of varying frequency; a firstoutput terminal connected to the junction between the two reactances; means for combining the voltage drops across the two reactances in like phase relationship; and a second output terminal connected to the combining means for deriving the combined voltage drops therefrom, whereby the voltage appearing across the two output terminals will be shifted in phase by a substantially constant angle of 90 throughout the range of variation of said varying frequency.

26. A network as in claim 25 in which the character of the two reactances is inductive.

27. A network as in claim 25 in which the character of the two reactances is capacitative.

28. In a phase shifting network for producing a substantially constant phase shift of ninety degrecs throughout a range of frequencies; a first series combination of a resistor and a conventional reactance; a second series combination of a resistor and a reversed reactance, and circuit means connected to the two series combinations and combining the phase shifts produced by the two reactances to provide a total phase shift of ninety degrees, the magnitude of said phase shift remaining substantially constant throughout said range of frequencies.

29. A network of the -olass described, comprising; input and output connections for said network and a transmission path extending therebetween; a first phase shifting means included in said transmission pat said phase shifting means producing a substantially constant predetermined angle of phase shift throughout a range of frequencies, and incidentally changing the relative magnitudes of the different frequency components of a complex wave at different frequencies within said range, thereby producing distortion; a feed back path extending from a first point on said transmission path Where said 23@ 1 phase shift and distortion arepresentada-secondpoint' on said'transmission path' hei'iw'eenE saidnet-A workinput connection and sai'drstpoint; and compensating meansineluded inA said feed'- hack path and comprising a second phase shifting' means for producing a total angle'o'f phaseshift of substantially 180 degrees throughout said range of frequencies in a loop including said feed back' path and that portionof said transmission path extending between said rst and second points; the circuit parameters of*y saidI coin'per'i'-av sating means being dimensioned to have transmission characteristics producing distortion simi-Y lar to and at'leastpartiall'y compensating for that producedby saidfirst phase shifting means'. l

30". Av network according to claim 2`9 in whichsaid substantially constant angle of phase shift is QOLdegrees and wherein said rst phase shift-M ing means comprises a transformer having at least one-windinga resistor'connected' to one-end of said winding and a capacitor connected to another point on saidv winding, the resistance v-aiu'erof said resistor being su'icient to maintain the impedance of a circuit including' said trans-v former winding, resistor and capacitorV substantialiy constant throughout said range of fre'- quencies 31. A network according to claim' 29, whereinY said' substantially constant angle of phase shift isf-gddegrees, amiA invwhich said first-,- phase shiftf" ing means includes a series circuit having conn'ectedtherein aVA resistor whose resistance iss'uiicie'ntly' large to render the impedance of the circuitsubstantiailvconstant throughout said'range" of frequencies, a. transformer anda capacitor,

and? means included in said transmission path* and" operatively associated with saidr transformer for deriving therefrom a voltage whose phase is shiftedby substantially' 90 deg-rees from thepha'se` v ofthe voltage applied to said series circuit, said'- amplier included therein, said feed back path ofV said network including saidfeed back path of said amplifier.

33. A network according to claim 31, wherein said second phase shifting network comprises at least one series circuit similar tosaid first-named series circuit, Asaid voltage deriving means being included inl said feed back path, and saidlast-k narned series circuit' being' connected in said feed back path to receive said applied Voltage therefrom.

FREDERICK W'. FRJINK.

References Cited in' the le of this patent UNITED' STATES PATENTS Number Name Date 11,915,440 Nyquist June 27, 1933' 2,020,327 Puington NOV. 12;, 1935" 2,174,166 Plebanski Sept. 26, 1939 2,248,045 Dow July 8, 1941 2,243,1321 Smith July 8, 1941' 2,341,322 Norton Feb. 8, 1944 2,369,066V Maxwell Feb. 6, 1945 2,392,476 Hodgson Jan. 8, 1946 2,412,995 Levy Dec". 24, 1946 2,468,302 Meacham Apr. 26, 194:9V` 

